Freelandville and Log Creek Purchases (Narrative) (Details) $ in Thousands		12 Months Ended
	Dec. 12, 2016 USD ($)	Dec. 31, 2016 T	Dec. 31, 2014 USD ($)	Mar. 22, 2014 USD ($)
Business Acquisition [Line Items]
Business acquisition purchase price | $			$ 311,453
Freelandville Purchase [Member]
Business Acquisition [Line Items]
Coal reserves acquired, tons		14,200,000
Purchase price | $				$ 18,250
Freelandville Purchase [Member] | Coal Supply Agreements [Member]
Business Acquisition [Line Items]
Coal reserves acquired, tons		1,435,000
Freelandville Purchase [Member] | Maximum [Member]
Business Acquisition [Line Items]
Coal sales agreement, monthly amount of allowable adjustment up or down		6,700
Log Creek [Member]
Business Acquisition [Line Items]
Business acquisition purchase price | $	$ 4,100
Log Creek [Member] | Coal Supply Agreements [Member]
Business Acquisition [Line Items]
Coal reserves acquired, tons		557,000
Sunrise Energy [Member] | Freelandville Purchase [Member]
Business Acquisition [Line Items]
Coal reserves acquired, tons		1,600,000